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Limit defining the weakest stimulus which can be sensed
In psychophysics, sensory threshold is the weakest stimulus that an organism can sense. Unless otherwise indicated, it is usually defined as the weakest stimulus that can be detected half the time, for example, as indicated by a point on a probability curve. Methods have been developed to measure thresholds in any of the senses. Several different sensory thresholds have been defined;
Absolute threshold: the lowest level at which a stimulus can be detected. Recognition threshold: the level at which a stimulus can not only be detected but also recognized. Differential threshold: the level at which an increase in a detected stimulus can be perceived. Terminal threshold: the level beyond which any increase to a stimulus no longer changes the perceived intensity.

The first systematic studies to determine sensory thresholds were conducted by Ernst Heinrich Weber, a physiologist and pioneer of experimental psychology at the Leipzig University. His experiments were intended to determine the absolute and difference, or differential, thresholds. Weber was able to define absolute and difference threshold statistically, which led to the establishment of Weber's Law and the concept of just noticeable difference to describe threshold perception of stimuli. Following Weber's work, Gustav Fechner, a pioneer of psychophysics, studied the relationship between the physical intensity of a stimulus and the psychologically perceived intensity of the stimulus. Comparing the measured intensity of sound waves with the perceived loudness, Fechner concluded that the intensity of a stimulus changes in proportion to the logarithm of the stimulus intensity. His findings would lead to the creation of the decibel scale. Different types of internal and external noise include excess stimuli, nervous system over or under stimulation, and conditions that falsely stimulate nerves in the absence of external stimuli. A universal absolute threshold is difficult to define a standard because of the variability of the measurements. While sensation occurs at the physical nerves, there can be reasons why it is not consistent. Age or nerve damage can affect sensation. Similarly, psychological factors can affect perception of physical sensation. Mental state, memory, mental illness, fatigue, and other factors can alter perception.